Contrast

Is noticeable differences exhibited when items are compared or set side by side.

An example of the radiographic property of contrast

Radiographic contrastRadiographic contrast

The difference in densities between light and dark regions on the radiographThe difference in densities between light and dark regions on the radiograph

The radiographic contrast of an image is the result of the interplay of subject contrast, film contrast, and scattered radiation

Radiographic contrast : the differences in densities .

Such densities variations are called radiographic contrast.

A radiograph that has marked differences in densities is a high contrast radiograph.

A radiograph that has less differences in densities is the a low contrast radiograph.

Is product of two separate factors: (1) Film contrast, is inherent in the film

and is influenced somewhat by processing of the film.

(2) Subject contrast, is determined by the size ,shape, and x-ray attenuating characteristics of the subject being examined and the energy (kvp) of the x-ray.

Patient factors1. Tissue thickness (age, sex, pathologic

change). 2. Tissue opacity (cellular composition,

tissue structure status of organ empty or filled.

3. Tissue densities (gas, fat, muscle, bone)

I. The difference between two adjacent densitiesII. Film displays high or low contrast characteristicsIII. Contrast controls slope of characteristic curve

log relative exposure

OpticalDensity

log relative exposure

OpticalDensity

LowerContrast

HigherContrast

Film latitude refers to the range of exposure values that will produce density in accepted diagnostic range (0.4 - 2.75).

Film Contrast Film Contrast Film processing: incomplete or excessive development; improper storage; light leaksFilm fog: contrast is reduced. Improper film processing and storage.

Scattered radiationScattered radiationPhotons that travel in directions other Photons that travel in directions other than that of the primary beam – fogging than that of the primary beam – fogging of the radiograph of the radiograph

Film with short latitude (high contrast) .Used in examination of an area of the body with poor subject contrast, in order to demonstrate higher radiographic contrast within the structures being examined.

Film with wide latitude (low contrast). Used in examination of an area of the body with high subject contrast, in order to demonstrate adequate numbers of densities within recorded image.

The useful densities recorded on the film are visible for comparison as ratio of densities.

The number of useful densities visible on film and the percentage of difference between them .

Low contrast : (long scale)the ratio of differences from one adjacent density

to an other is slight (the total number of useful densities would be maximal).

High contrast : (short scale)the ratio of differences between densities are

considerable (minimal number of densities ).

(1) Kvp/ mAs relationship.High contrast: high mAs +low kvpLow contrast: low mAs +high kvp

(2) Fog levels.

(3) Intensifying screen.

CONTROLLING FACTORS?

INFLUENCING FACTORS?

Subject Subject Signal Signal Exposure energy Exposure energy (kvp) (kvp) Receptor Receptor (Film) (Film) Processing Processing (development)(development)

Contrast sourcesContrast sources

Causes of radiographic fog.

Type and constitution of developer. Temperature of developer. Freshness of developer. Agitation of the film in the developer.

1- Kvp :to penetrate 2-mAs :to maintain

density.

75 kvp 79 kvp 79 kvp

Kvp :to penetrate

mAs :to maintain density.

1. Small extremities 602. Large extremities 803. Skull 854. Sinuses 705. Abdomen, pelvis and posterior spine 756. Lateral vertebrae 857. GI tract 908. Chest 809. Chest lateral 9010. Chest oblique 85

Density is determined principally by mAsAs shown by these radiographs of abdomen taken at 70 kvp. A,10mAs.B, plus 25 %, 12.5 mAs C, plus 50 %,15 mAs.

High and low mAs effect..

Scatter radiation negatively affect contrast. Scatter radiation fog the film(making it

Gary) and reduces the contrast. The more scatter there is the lower the

contrast You can reduce the amount of scatter

radiation by lowing kvp. Kvp has to be high enough to penetrate.

the part .

Reduce formation of scatter;1. Used of compression.2. Beam Restriction devices.3. Used of low kvp. Prevent scatter to reach the film;1. Air gab technique.2. Used of lead blocker3. used of x-ray grids.

A. Tissue1. Volume.2. Thickness.3. Opacity. B. object-film distance

When tissue compressed, scatter radiation is reduced resulting in lower dose and improve image contrast (reduced fog)

Beam Restriction devices1. Diaphragms2. Cones3. Collimators4. Lead blockers

A device used to reduce intensity of scatter radiation in remnant x-ray beam.

• Grid ratios : are the maximum degree of angling before cutoff.

• Grid ratios compares the height of lead strips to the distance between each strip

Number of lead strips per inch.

When to use a grid ?

Over 10 cm thickness. Above 60 kvp